The present invention relates to a method for planning the positioning of a ball joint prosthesis, on the basis of the position and/or orientation of areas where the cup of the joint prosthesis abuts the bony structure of the joint socket. The present invention is in particular directed to a method for planning the positioning of a hip joint prosthesis. The present invention is also directed to a method for optimising the positioning of a ball joint prosthesis, in particular a hip joint prosthesis. Such prostheses usually comprise a cup which is placed in the pelvic bone and a stem which is placed in the femur, wherein the ball-shaped head coupled to the stem can be moved within the cup.
Although the present invention is described in connection with hip joint surgery, the present invention can also be applied to any surgery on a ball joint, such as for example a shoulder joint.
Within hip replacement surgery, it is difficult to define an optimum cup position with regard to interference with anatomical structures. Until now, most surgeons have placed the cup according to the classical Lewinnek safe zone (40°±10° inclination, 15°±10° anteversion), disregarding the interaction between the femur and the cup implant. Emerging techniques address this problem using a so-called femur-first surgical approach.
Within such an approach, the femur is prepared in a first step and the position of the (trial) implant is then determined. Based on this knowledge, the cup implant is positioned in a way which approximately optimises the post-operative range of motion. In most of the techniques, arithmetic formulae have been developed which optimise the range of motion (ROM) according to purely implant-implant impingements under certain assumptions (for example, a fixed CCD angle, femoral tilt, etc.). This approach is described in US 2008/294265. Simulations of post-operative ROMs are described in US 2008/287962 and also in U.S. Pat. No. 5,880,976, US 2006/241405 and US 2007/066917.
A particular goal for optimum cup placement is to determine an appropriate anteversion of the cup. One suggestion by Archbold et al. (The Relationship of the Orientation of the Transverse Acetabular Ligament and Acetablular Labrum to the Suggested Safe Zones of Cup Positioning in Total Hip Arthroplasty. Hip International, Vol. 18, No. 1, January, 2008, pp. 1-6) is to use the transverse acetabular ligament (TAL) as an aid. It has been shown that TAL can reduce dislocations and thus functionally optimise the cup position. US 2008/0255584 also suggests using this anatomical structure for navigation purposes.
In all of the above-mentioned prior art, ROM aspects are essentially addressed and the cup position/orientation is optimised for this purpose. However, a purely ROM-orientated optimisation disregards other important biomechanical aspects. None of the approaches described above includes a systematic optimisation according to such multiple criteria. This can significantly influence the biomechanical stability of the individual artificial joint.
In addition, formula-based approaches essentially use formulae for combined anteversion, i.e. the (weighted) sum of the cup anteversion and the stem antetorsion are to have a certain value. In this case, only implant-to-implant impingement can be addressed in a generic setup. Such approaches are based on many assumptions about the implant position and disregard other influences such as the CCD angle, femoral tilt or varus-valgus deviation.
The TAL approach only considers very limited information. It could be used as an additional feature within navigation procedures, but does not in itself provide a method for functional optimisation according to different significant features.